The present invention may be usefully applied to all metallic wires, including non-ferrous metals, but it is especially directed to aluminum and aluminum alloys. Improved aluminum alloy electrical conductors were developed and improved throughout the 1970's including Triple-E.RTM. for building wire and alloy 6201 for overhead conductors among many others. The especially useful properties of the various alloy conductors are generally achieved through combinations of working the metal and thermal treatments. It is to the latter manufacturing operation that this invention is directed.
Of the various thermal treatment by which the alloy rod or wire properties are achieved, this invention is most useful in the annealing of wire, but may also be used in homogenization, solution heat treating (or solutionizing), and precipitation treating (or aging) of wire or rod.
Annealing, the thermal treatment for which one embodiment of the present invention is particularly appropriate, is the heating of the alloy to about the recrystallization temperature and maintaining the desired temperature for a particular desired period of time, after which the alloy is cooled or quenched. Annealing is often used to soften metal by removal of stress resulting from cold working or by coalescing precipitates from solid solution.
Homogenization is a high temperature soaking treatment to eliminate or reduce segregation by diffusion, obtaining thereby a uniform structure and an even distribution of alloying constituents; it has been described as particularly applicable for those aluminum alloys having up to 12% alloying constituents. Often, homogenization consists of heating to near the eutectic melting point and maintaining this temperature for up to several hours. A stronger, more ductile (and homogeneous) structure may result if homogenization is properly performed.
Generally, the term "solution heat treating" is applied when an alloy is heated at a given temperature for a given time in order to allow soluble constituents to enter into solid solution, where they are retained in a supersaturated state after quenching. A solution heat treated aluminum alloy, suitably quenched and with subsequent treatments, can provide high mechanical properties such as tensile strengths greater than 90,000 lbf/in.sup.2 and shear strengths of 50,000 lbf/in.sup.2.
Precipitation treating, or aging, is of two types, natural (usually at room temperature) or artificial (usually at elevated temperatures). Aging gives certain alloys maximum strength and may be performed in coordination with certain solution heat treatment procedures. Aging comprises the precipitation of small particles from solid solution under controlled temperatures.
Various non-ferrous and especially aluminum products are used as electrical conductors, including EC (electrical conductor grade) and various alloys including Triple E.RTM., Super T.RTM., NiCo.RTM., the Aluminum Association 1350, 5005, 6101, 6201 and others. Electrical conductivity standards from about 50 percent of IACS (International Annealed Copper Standard) to about 62% of IACS are common, depending on the alloy and use intended. Certain alloys, for example the proprietary Triple E aluminum conductor alloy, require careful preparation to achieve their most desirable properties. Ordinarily, high iron aluminum alloys may be manufactured continuously and certain production economies associated therewith are obtained; see for example U.S. Pat. No. Re 28,419 (reissue of U.S. Pat. No. 3,512,221) and others of this family.
In producing many of these aluminum alloys, and especially the high-iron aluminum alloys such as Triple E, a continuous casting machine serves as a means to solidify the molten aluminum alloy metal into a cast bar product which is subsequently hot-formed into an elongated rod or other intermediate product. The hot forming may be used to impart substantial movement to the cast bar along a plurality of angularly disposed axes. For illustration but not limitation, the casting machine may be of the wheel/band type including a casting wheel having in the periphery thereof a groove partially closed by an endless band. The wheel and band cooperate to provide a mold, into one end of which molten metal is poured to solidify, and from the other end of which the cast bar is emitted in substantially that condition in which it was solidified. The case bar is often conveyed directly into a rolling mill.
The rolling mill is of a conventional type having a plurality of roll stands arranged to hot-form the cast bar by a series of deformations. By rolling the cast bar substantially immediately upon extraction from the casting machine, the cast bar remains at a hot-forming temperature within a range of hot-forming temperatures. The case bar may, however be adjusted by thermal treatment, as desired, by appropriate apparatus. The rolling mill reduces the bar cross section and elongates it to produce a rod product having a smaller cross section.
Rolled alloy rod such as is produced according to the foregoing is then processed in a reduction operation designed to produce continuous lengths of wire having various diameters, such as by drawing operations. Such drawing includes passing the rolled rod product through a successive series of progressively constricted dies to form the wire of desired diameter. Alternatively, the rod may be rolled down to smaller (wire) diameters. At the conclusion of the cross section reduction process, and intermediately during the process with certain alloys, the wire product is usually subjected to annealing or one of the foregoing thermal treatments to achieve a desired combination of properties. With certain alloy conductor materials such as Triple-E, the unannealed rod (F temper) is cold drawn without intermediate anneals, resulting generally in a wire product having very high tensile strength, and low conductivity and/or ultimate elongation properties.
Annealing may be performed in a batch furnace, or continuously, as by electrical resistance annealing, induction annealing, convection annealing, or radiation annealing. Among these thermal treatment operations, in-line continuous annealing is the most productive and energy efficient if carefully performed. Electrical resistance annealing, if possible, would be the most effective and easiest to accurately control, as is necessary to accommodate variations in line speed. Conventional apparatus, however, are unequal to the task as either unacceptably long catenaries of wire must be heated, unacceptably slow line speeds, or incompletely annealed wire results. Wire breakage due to high electrical current levels is a continuing problem at the high line speeds necessary for economic thermal treatments. This is especially true at line speeds above several thousand feet per minute or with small diameter wire.